The invention relates to a device for monitoring the operating condition of a gravimetric measuring instrument, specifically a balance. The balance for which the inventive monitoring device is proposed has a weighing cell and an electronic weighing circuit arrangement with a signal-processing device, a memory device and a time clock, an output unit, as well as at least one electronic inclination sensor that is connected for communication with the signal-processing device.
Balances and other gravimetric measuring instruments, for example instruments for the gravimetric determination of moisture content or thermo-gravimetric instruments, constitute a class of instruments that have to meet very particular requirements in regard to their setup in the place where they are used. A balance that is set up for example in a laboratory must be positioned in such a way that the load receiver of the weighing cell of the balance is aligned exactly in the direction of the force of gravity. If the alignment deviates from the direction of the gravitational field, the result of weighings performed on the balance will be short by a factor that represents the cosine of the angle of inclination, i.e., the angle by which the alignment of the load receiver of the weighing cell deviates from the direction of the gravity force. In a precisely assembled balance, the same angle also represents the deviation of the seating surface of the weighing cell from the horizontal direction. The position where the load receiver is perfectly aligned with the direction of gravity may also be referred to as the reference position of the balance. For balances that are subject to official certification, it is therefore often necessary to provide a leveling device and an inclination sensor, for example a spirit level which consists of a container that is preferably made of glass and partially filled with a liquid, so as to leave a gas bubble. The leveling device is preferably incorporated in the supporting feet of the balance through an arrangement where the respective heights of at least two of the supporting feet can be varied by means of height-adjustment screws.
The known state of the art includes different inclination-measuring devices that register the inclination angle of a balance at its place of installation electronically. For example, DE 32 34 372 A1 describes different versions of electronic inclination-measuring devices such as a spirit level with an optical sensor, a capacitive inclination-measuring device, or a pendulum that is equipped with a strain gauge to determine its deviation from the vertical position. The measuring signal of an electronic inclination-measuring device of this type is used to compensate inclination-related weighing errors and thus to ensure a correct weighing result even if the balance is not in exact alignment with the direction of gravity, particularly in the case where the inclination of the balance is dependent on the weighing load placed on the balance.
A balance with an electronic inclination-measuring device is disclosed in JP 61 108927 A2 where the inclination-measuring device consists of a container that is partially filled with an electrically conductive fluid, leaving an air bubble. An inclined position of the balance is detected through the change in electrical resistance, and an acoustical alarm is triggered if the balance is found to be too far out of level.
The inclination-monitoring device for a balance described in FR 2 639 111 A1 is capable of selectively detecting out-of-level positions relative to horizontal x- and y-directions. This inclination-monitoring device has at least one inclination sensor, an electrical circuit with an electrical power supply for the inclination-monitoring device, as well as a relay switch which causes the system to block the weighing function when given limits of the inclination are exceeded. As an alternative possibility, the display of the weighing result or its transfer to peripheral instruments can be blocked. Furthermore, it is possible to indicate the out-of-level condition of the balance as well as the direction of the slope gradient by way of light-emitting diodes.
Especially in weighing processes that are relevant to product quality, it is required procedure to check the spirit level before starting the actual weighing process in order to verify that the balance is properly leveled. However, this rule is not always followed in practice. If a user of the balance notices at any time that the balance is no longer in its reference position, he is confronted with the question whether the deviation is relevant, i.e., by how much the actual position deviates from the reference condition and for how long the deviation has been present. Subsequently, the magnitude of the potential consequences has to be assessed.
Even if an alarm is triggered in a balance when the out-of-level condition exceeds given limits, it is not necessarily evident to the user of the balance how far back in time the balance was put out of its reference position and what caused the out-of-level condition. However, the so-called GLP (Good Laboratory Practice) guidelines mandate traceable proof of the reliability of the weighing result. It is therefore necessary to know the point in time at which the weighing result ceased to be reliable. An automatic correction of the weighing result in regard to an electronically detected out-of-level condition of the balance is not always desirable, because the deviation from the leveled position could in some cases deteriorate far beyond a justifiable tolerance limit.